Currently one of the preferred arrangements of a combined cycle power plant is a single shaft arrangement. Both the gas turbine and the steam turbine are arranged on the same shaft and drive a common generator. The major driver in favor of this arrangement is the cost benefit stemming from the fact, that only one large generator is needed.
Future plants will be significantly bigger than current single shaft power trains and we will be confronted higher steam parameters, i.e. higher pressures and temperatures. As a result, several design parameters change and the optimum configuration may differ from the current single shaft arrangement.
One of the problems of increasing the size is the steam lines, which will be correspondingly thicker. In addition with a tendency towards increased cycle parameters advanced and more costly steels must be employed, which again drives the cost per unit length of the steam line up. A new configuration, which minimizes the length of steam piping, would be advantageous.
Further, in a single shaft arrangement, the height of the shaft line is dictated by the length of the last stage blade of the low pressure steam turbine. The costly foundation of the gas turbine and an attached generator is therefore higher than actually needed considering gas turbine requirements alone.
In addition, the highly efficient air cooled generators, which are commonly used for single shaft power trains have a technology limit in terms of size, which implies that single shaft plants for future gas turbines would need to employ a less efficient and more expensive (first cost versus rated power) water cooled generator. Therefore todays cost benefit of the single shaft arrangement is deteriorating.
Both arguments have led to considerations to employ a multi-shaft configuration for the next generation of larger gas turbines. Conventionally the steam turbine and its own generator are placed parallel to the gas turbine axis at an offset. These arrangements are for example known from the U.S. Pat. No. 5,649,416. The two turbines are positioned such that they can be served with one and the same crane. The steam lines, which span a rather long axial distance in the case of the single shaft arrangement, now span a shorter axial distance in the multi shaft arrangement. However, the steam lines need to cover additional lateral distance, such that the overall length remains comparable to the length in a single shaft power arrangement. In consequence known single and multi-shaft combined cycle power plants will face considerable challenges with respect to arrangement, piping materials and costs.